Note: Descriptions are shown in the official language in which they were submitted.
~57
COAL LIQUEFACTION DESULFURIZATION PROCESS
Background of the Invention
,
This invention pertains to desulfurization of
solvent refined coal liquefaction products.
As ~ackground to the present invention, U.S.
Patent 4,077,866 - Owen et al appears to he of primary
interest. It proposes a solvent coal refining process
in which the coal slurry is desulfurized by contact with
a solid sulfur scavenger, such as iron (but which may
~6~57
include any of numerous other materials, some of which
are also ~isclosed herein). The inclusion of,the sulfur
scavenger in the solvent-coal sl~rry, in accordance
with the process of the Owen et al patent, differs from
S the present invention in that Owen et al would require
sufficient scavenger to react with substantially all of
the sulfur present. This includes volatile low mole-
cular weight sulfur compounds and hydrogen sulfide.
Indeed, with regard to one example, the Owen et al
patent states that (following desulfurization) no gase-
ous hydrogen sulfide was evolved (Col. 9, lines 55-56).
Other patents considered as background to the
present invention include U.S. 3,284,345 - Ishiko et al;
U.S. Patent 2,697,064 - Brown; U.S. Patent 738,656 -
Burwell et al; U.S. Patent 1,587,491 - Cross; U.S.
Patent 3,063,936 - Pearce et al; U.S. Patent 3,769,197 -
Leas et al; and U.S. Patent 4,190,518 - Giannetti.
Ishiko et al teach desulfurization of crude oil
or heavy oil by contact with a particularly reactive
forrn of reduced iron powder. A process of this general
nature, as related to petroleum fractions, is also
referred to in the background portion of the Brown
patent.
~6525~7
Other sulfur - reactive reagents are used for
desulfurizing vapor phase petroleum products ~ccording
to the processes disclosed in the Burwell et al and
Cross patents.
A more complex desulfurization process for
hydrocarbon oils, such as petroleum fractions, ~ut
including some of the same sulfur reactants included in
the disclosure of the present invention, is seen in U.S.
Patent 3,063,936 - Pearce et al.
Finally, U.S. Patents 3,769,197 - Leas et al and
4,190,518 - Giannetti et al ~oth pertain to solvent
refined coal desulfurization processes, wherein sulfur i5
extracted by reaction in the vapor phase. Coinci~en-
tally, the solvent coal slurry in the Giannettti et al
process is hydrogenated (and any sulfur compounds
present probably converted to some other form) in the
presence of a hydrogenation catalyst, which is chosen
fro~ a wide ranqe of materials including many compounds
similar to those referred to herein as sulfur getters.
Notwithstanding these prior processes, there
remains a continuing need for more efficient means for
desulfurizing solvent refined coal products, and par-
ticularly the non-volatile portions thereof. Because,
non-volatile sulfur compounds in solvent refined coal
products tend to be high molecular weight multi-cyclic
~L~6~.~S~7
--4--
anthracene and phenanthrene-type compounds, such com-
pounds are somewhat more difficult to remove t~an other
sulfur compounds. In high boiling point solvent refined
coal fractions, these heavy sulfur compounds may com-
prise on the order of 1% by weight of the product.However, reduction of sulfur content in these fractions,
by as little as a tenth of a percent, may be signifi-
cant in some circumstances.
It is therefore the general object of the
present invention to provide a solvent coal refining
desulfurization process improved with respect to sim-
plicity and efficiency of sulfur capture as compared to
prior known processes.
1~5'Z5~
Brief Description of the Invention
In one particular aspect the present invention provides
an improved process for preparing desulfurized solvent
refined coal liquefaction products comprising the steps of:
(a) forming a slurry of comminuted coal and a hydrogen
- donor solvent,
(b) reacting said slurry with hydrogen at elevated
temperature and pressure to dissolve a portion of said coal
and to form a reacted mixture comprising said slurry, gases
and a solution containing volatile and non-volatile solvent
refined coal products,
(c) separating said gases and said volatile products
having an ambieht pressure boiling point below about 450F
from said reacted mixture to form a devolatilized reacted
; mixture,
(d) desulfurizing the remaining portion of said devolatilized
reacted mixture by contacting the same with a solvent insoluble
sulfur getter, for a time and under conditions sufficient
for said getter to react with sulfur -ln said devolatilized
reacted mixture to form getter-sulfur solids and desulfurized
solvent refined coal products herein, and
(e) separating said getter-sulfur solids and othel
insoluble solids remaining in said devolatilized reacted
mixture from said desulfurized solvent refined coal products.
The getter may be combined with the reacted solvent-coal
mlxture in the form of a getter slurry, utLlizing the same or
a compatible hydrogen donor solvent. The getter, a particulate
soLid to begin with, reacts to form a solid getter-sulfur
compound, removable with other insoluble components of the
slurry, in a conventional solids removal step.
~ 6~ 7
In general, a getter reaction time of up to 60
minu~es may be re~uired and the proportion of`getter
used is on the order of 1-10% by weight, based on the
weight of reacted coal in the devolatilized reacted
coal-solvent slurry. Agitation or transport of mixture
through tubular reactors may be utilized to effect more
eEficient reaction with the getter material.
The process parameters for reaction time, pro-
portion of getter, and process conditions will of course
vary over a wide range depending on the relative re-
activity of the getter, the degree of desulfurization
required, and the characteristics of the coal feed.
Obviously then, reaction time and temperature-pressure
. conditions will be selected to effect the desired degree
of desulfurization in any specific situation. In all
cases, however, it is expected that the sulfur getter
reaction will function most effectively at elevated
temperature. It is therefore highly preferred that the
present invention be incorporated in a coal liquefaction
process in which the getter is added to a liquid mixture
already at elevated temperature, preferably above
. 300F.
Materials which ~ay be used as sulfur getters, in
accordance with the present invention, include iron, and
., ,
~G~2~i7
iron compounds such as iron oxiae (both ferrous and
ferric forms) and ferrous carbonate, includin~ mineral
forms thereof such as siderite. Other getters include
manganese, nickel, calcium, zinc, lead, and compounds
incl~d~ng these elements, particularly including oxides
and carbonates thereof, and, in the case of calcium,
limestone ~typically calcium carbonate or a mixture of
calcium and magnesium oxide and carbonate). Of these
possible getter materials, metallic iron is presently
preferred.
Brief Description of Figure
The accompanying Figure comprises a schematic
illustration of a coal li~uefaction desulfurization
process in accordance with the present invention.
Detailed Descriptic"~ of the Invention
For a better understanding of the present inven-
tion, reference may be made to the following detailed
description thereof, taken in conjunction with the
accornpanying Figure, and the appended claims.
Referring more specifically to the Figure, there
is shown a process wherein coal feed 4, in finely
divided or comminuted ~orm, is combined with a hydrogen
donor solvent in slurry mixer 5. Such a solvent may
~G5~57
.
comprise, for exampler tetrahydronapthalene, partially
hydrogenated phenanthrenes, creosote oil, hydrogenated
creosote oils, or process recycle streams having similar
solvent characteristics (or combinations of the fore-
going). In the embodiment of the invention shown, arecycle oil stream 3~, at elevated temperature, provides
process solvent and heat for the coal-solvent slurry.
Temperature in the mixer may be from ambient to 450F.
A separated solids recycle stream 49 is also introduced
~0 into slurry mixer 5.
Slurry from mixer 5, is supplemented by a
hydrogen-rich gas enrichment stream 9 to form a lique-
faction slurry feed stream 8 which is heated by pre-
heater 10. The heated slurry feed 15 is then passed to
liquefaction reactor 18, in which additional fresh
hydrogen-rich gas 17 is introduced. In reactor 18,
elevated temperature and pressure conditions, on the
order of 300-5000 psig and 500-900F, are maintained.
~nder these conditions, solid bridges in the coal ~atrix
are thermally broken and resultant carbonaceous products
are dissolved in the solvent. The reacted solvent-
coal slurry 20 from liquefaction reactor 18 is passed
into a conventional separator 26, such as a multi-stage
flash evaporator operated at temperature and pressure
conditions selected generally to remove volatile com-
ponents having an ambient pressure boiling point below
,
,
5~57
g
45~F. Preferably gas separator 26 operates at the
pressure of reactor 18 and from 300F up to within 25F
of the outlet temperature from reactor 18.
Individual streams which may be separated, for
S example, are a hydrogen-rich stream 23, a hydrogen
sulfide-rich stream 24, and a stream 25 consisting
predominately of carbon oxides and low molecular weight
hydrocarbons. These streams may also be treated for
sulfur or sulfur compound removal bv conventional gas
treatment technology.
The remaining devolatilized reacted solvent-
coal stream 27 proceeds to a sulfur getter reactor zone
29. There the mixture, at a temperature above 300F, is
contacte~ for up to 60 minutes with a "sulfur getter".
A "sulfur getter" is a sulfur-reactive solid material,
such as particulate metallic iron, preferably in slurry
with a solvent, wherein the solvent may consist of
additional hydrogen donor solvent, the same as or com-
patible with that used in slurry mixer 5. Getter-slurry
stream 30, reacted with devolatiled reacted solvent-coal
slurry stream 27, forms an intermediate product stream
31.
Effective and practical sulfur capture in re-
actor 29 req~ires maintenance of an elevated ternperature
there. Preferably, this results inherently from the
"
525~
--10--
heat input of devolatilized, reacted solvent-coal stream
27 r the normal process temperature of which is well above
300F. The te~perature and time of reaction in sulfur
getter reactor 29, preferabIy above 300F and up to 60
minutes, is maintained so as to effect the desired degree
of desulfuri7ation therein. In sulfur getter reactor 29,
the sulfur getter forms a getter-sulfur compound which is
also solid. Interrnediate product stream 31 is then for-
warded to a vacuum distillation stage 37, ~herein a re-
cycle oil stream 38 (having an ambient pressure boilingrange of 450-900F) is removed and recycled as process
solvent to slurry mixer 5. A further gaseous component
strearn 39 (a light distillate fraction boiling up to
450F) is evolved and separated and the rernaining higller
boilin~ point co~ponents are forwarded to a multi-stage
solid separator systeM 44, which may consist o~ critical
solvent separation or fractional phase separation syste-ns,
centrifuges or filters, wherein one or more separate solid
carbonaceous product streams 46 and 48 are removed. One
suitable solids separation system is a critical solvent
- de-ashing process as disclosed in U.~. Patent 4,119,523.
In addition, a mineral matter and unconverted maceral-rich
residue stream 45 is also separated.
A portion 49 of ash- and mineral residue-free
carbonaceous product in separator 44 may be recycled to
the slurry mix zone 5.
v
~6~i7
Alternatively, solid separator system 44 may
precede the vacuum distillation stage 37 in order to
allow the ~se of a filter, centrifuge or other solids
separations technique to remove the mineral solid residue
before subjecting the de-ashed filtrate to the vacuum
distillation stage 37.
In general, the process invention disclosed here
relates to an improved liquefaction process by which
coal can be effectively converted to a low ash and low
sulfur carbonaceous material, referred to generically as
"solvent refined coal". It can be used as a fuel in an
environmentally acceptable manner without costly gas
scrubbing equipment.
Conventionally, coal is slurried with a hydrogen
donor solvent, sometimes referred to as a pastiny oil,
passed through a preheater, and then through one or more
dissolvers, in the presence of hydrogen-rich gases, at
elevated temperat~re and pressure.
- In accordance with the present invention, this
reactor effluent is devolatized to remove, among other
things, low molecular weight compounds and hydrogen
sulfide gas. To the remaining slurry reactor effluent
is added a sulfur getter material, preferably metallic
iron. Solids, including the reacted getter, plus
. .
~65'~
mineral ash and unconverted coal macerals are then
separated from the condensed reactor effluent`
Sulfur getters have lo~g been known to entrap
sulfur from sulfur bearing hydrocarbonaceous liquids
incl~din~ pe~L-oleurn liquids and coal liquids. However,
sulf~r getters tend to react preferentially with rnore
reactive sulfur compounds particularly including hydro-
gen sulfide and low molecular weight organic sulfur
compounds.
In accordance with the present invention, the
low molecular weight sulfur organic compounds and hydro-
gen sulfide, which would otherwise react preferentially
with the getter material, are removed by a gas stripping
or devolatilization stage, prior to reaction with the
getter material. Sulfur compounds remaining in the
~evolatilized mixtures tend to be rnore cornplex and
higher molecular weight organic-sulfur compounds. And
it is these compounds which remain availahle for re-
action with the getter material in the resultant reacted
solvent-coal mixture.
A sulfur ~getter" functions by combining with
sulfur to form a tightly knit chemically bound sulfur
cornpound, subsequently removable in the process of the
present invention with other solid rnaterials such as
2~ ash. Among known getter materials, iron is perhaps best
~65~57
-13-
known. In metallic for~ or in the form of an~oxide or
carbonate, it readily combines with sulfur compounds to
for~ iron sulfides.
Other metals also known to be sulfur getters are
manganese, nickel, calcium, zinc, and lead. These metals
also function either in metallic form or as a metal
oxide or cabonate. In some cases, such as zinc
chloride, metal halides, particularly chlorides, are
also effective. In the case of iron, either the ferrous
or ferric compounds will readily form iron sulfide.
Minerals containing sulfur-reactive metal, such as iron,
; also function as getters. Examples of such metal
compounds include the mineral siderite, which contains
FeCO3 an~ limestone, comprised largely of calcium
carbonate.
.
In the process of the present invention, the
getter materials can be used alone or as combinations
and are preferably used as fine powders having par-
ticulate sizes less than 14 mesh (Tyler Classification
System) in size. These po~7ders may he mixe~ with pro-
cess solvents or other suitable vehicles in which to
suspend the powders for intro~luction into the process
stream.
In some cases, the sulfur ~ettering action may
be activated or enhanced by reaction of the getter with
~5Z~7
-14-
hydrGgen. For that purpose, a very slow hydrogen flow
may be passeA through the sulfur getter holdi~a tank to
improve the efficiency of the su~fur capture by the
getter.
In a typical solvent-coal liquefaction process,
solld bridges holding the framework of coal intact are
thermally broken and free radical sites thus generated
are terminated by a hydrogen donor solvent. In the
dissolution process, water and H2S are formed in
abundance. To some degree, this results from cleavage
of heteroatoms containing sulfur in the coal.
Liquefaction occurs rapidly with many coals,
often in a matter o~ minutes. However, longer residence
times are necessary to significantly reduce the sulfur
content typically contributed by heteroatoms in the
higher boiling point components of the solvent refined
coal. This longer residence time, while it may also be
dictated by other process parameters, is generally
undesirable not only because it would normally entail a
lar~er reactor, but also because it is accompanied by
higher hydrogen consumption rates, higher residual
yields, lower hydrocarbon gas concentrations in the
product.
Coal, suitable for conventional processes of the
type adopted for use of the present invention, is
Z57
--15--
generally that of a rank lower than anthracite, such as
bituminous, sub-bituminous, or lignite, or mixtures
thereof. The coal may be used directly from the mine
or may be precleaned to remove a portion of the entrain-
ed mineral matter. In any event, solid feed material isgenerally ground to a size typically less than 8 mesh
- (Tyler Screen Classification), or more preferentially
less than 20 mesh, and dried to remove substantial
; moisture to a level for bituminous or sub-bituminous
coals of less than 4 weight ~. The concentration of
coal in the slurry may vary from 20 to 55% by weight.
In the slurry mix tank, the mixture must be maintained
at elevated temperatures to keep the viscosity of the
solvent low enough to pump it and sufficiently high so
that moisture entrained in the feed coal will be re-
moved. For reaction, the slurry is pumped up to pres-
sures on the order of 300-5000 psig and the slurry is
mixed with a hydrogen-rich gaseous stream at a ratio of
from 10,000-40,000 standard cubic foot per ton of feed
coal. The three phase gas slurry stream is then in-
troduced into a preheater system, which may consist of a
tubular reactor, and the three phase mixture, with its
temperature increased to the order of 600-850F, prefer-
ably to a maximum of 800F, is introduced to one or more
dissolver vessels, typically tubular reactors operated
in an adiaba~ic mode.
3L~G5Z57
-16-
In the preheater section, the viscosity of the
slurry changes as the slurry flows through th~ tubes and
coal is dissolved, forming intially a gel-like material
which shortly therea~ter diminishes sharply in viscosity
to a relatively freely flowing fluid. Upon entry into
the dissolver, other changes occur. These changes
inclufle f~rther dissolution of the coal and li~uid,
hydrogen transfer from the solvent to the coal, rehydro-
genation of recycled solvent, removal of heteroatoms ~S,
N, oxygen) from the coal and recyclefl feed, reduction
of certain components in the coal ash, such as FeS2 to
FeS, and hydrocrackin~ of heavy coal liquids. To some
degree, the mineral matter in the coal may catalyze the
above reactions.
Upon exiting the flissolvers, the solvent-coal
mixture is generally separatefl through several stages in
which the pressure is droppefl in a stepwise manner
~iving rise to overhead streams successively enriched in
higher boiling point components. The lower hoiling
point effluents are treated in a gas handling system
wherein ultimately the vapors are cooled and let down in
pressure to recover the light gases, water, and organic-
rich condensates. The separation, collection, and gas
pu~ification steps may be accomplished in a gas treat-
ment area where the overhead from each separator iscombined. The ~arie~y of methods available for gas
separati~n and handling are well known to those skilled
1~525~7
.
in the art. In any event, it is in these gas separation
stages of the separator, handling effluent frdm the
dissolvers, that low molecular weight sulfur compounds
and hydrogen sul~ide are generally removed, prior to
gettering in accordance with the present invention.
Depending on the process, t~e solvent coal
mixture in the dissolvers may be remixed with fresh
hydrogen and injected into additional dissolver vessels
for further reaction. Effluent from this second or
downstream dissolver is also flashed for removal of
lower boiling point components.
In general, the light gases removed include
hydro~en, hydrogen sulfide, carbon monoxide, carbon
~ioxide, nitrogen, water, and Cl-C4 hydrocarbons.
These gases may be scrubbed to remove acidic or alkaline
components in the hydroyen stream and the hydrogen and
lower hydrocarbons may be recycled in various stages in
the process or may be burned for fuel.
In accordance with the present invention, the
remainin~ effluent, consisting of liquid/solid slurry,
is then contacted with the sulfur getter, preferably
containe~ in a slurry with additional process solvent.
The combination of separator underflow plus
sulfur scavenger (getter) may in some cases require a
holding time to allo~ adequate reaction to occur. In
Z57
-18-
this reaction-holding process, materials may be held in
a reaction vessel for any desired length of time to
achieve a desired degree of desulfurization. The ef-
fluent from this holding vessel is then passed to an ash
separation system from which residue is rejected and the
contaminant mineral and solids-free solvent refined coal
materials are obtainefl. If desired, part of the
effluent from the separator may be passed directly to
the solids separation unit without having to pass
through a stage where a sulfur getter is employed.
In one embodiment of this process, the effluent
stream from the holding vessel may be fed directly to a
vacuum distillation tower prior to solids separation.
As previously indicated, the primary advantages
of the present invention are that by the prior removal
of low boiling point components, particularly including
~12S and low molecular weight sulfur compounds, the
sulfur getter is more effectively utilized to remove
sulfur heteroatoms and higher molecular weight sulfur
compounds, rather than the low molecular weight sulfur
compounds and hydrogen sulfides which would otherwise
preferentially react with the getter. An additional
advantage of the present process is the ease of removal
of the solids sulfur-getter compound in the process.
This permits removal of the sulfur by-product in the ash
~;5'Z57
.
--19--
separati~n step. The additional solids load on the
separation step is minimal compared to the galn realized
in reducing the sulfur content.
Still another advantage of the present invention
is the utilization of the process temperature of the
reactefl solvent-coal mixture to effect sulfur getter-
ing without the necessity of any additional heating or
reheating. ~n this respect, the temperature of the
reacted solvent-coal mixture in conventional practice,
usually in the range 300-700F, is entirely suitable
for the sulfur gettering action in accor~ance with the
present invention.
While this invention has been described with
respect to specific embodiments thereof, it is not
limited thereto. The appended claims arè intended to be
construed to encompass not only the forms and embodi-
ments of the invention flescribed but to such other forms
and embodiments as may be devised by those skilled in
the art, which forms and embodiments are within the true
spirit and scope o~ the present invention.
